1. Field of the Invention
The subject disclosure relates to systems and methods for positioning objects, and more particularly, to an improved system and method for actively aligning an optical fiber to a laser diode, a fiber array to an optical waveguide, and the like.
2. Background of the Related Art
High-accuracy positioning stages have many industrial and scientific applications. Conventional translation devices with one degree of freedom comprise a mobile platform sliding on a base frame. The range of movement determined by typical sliding, or kinematic guides is controlled by an actuator, whose body is fixed to the base frame and whose mobile part is connected to the platform by an appropriate transmission means. When the movement of the actuator is approximately rectilinear and parallel to the translational axis of the guide, the transmission means is often no more than the platform being supported against the actuator through an intermediate ball. The retention of such a platform against the intermediate ball is generally ensured by a spring stretched between the frame and the platform. Two well known actuators are manual micrometer screws and piezoelectric disk stacks.
Many applications require several axes of motion. Multi-axis motion can be obtained by stacking several stages. For example, such stacking may take the form of bolting the top plate of one bearing stage to the base of another linear stage so that the respective positioning axes are at right angles to each other to produce a two-axis stage, or so-called “xy”-stage. Moreover, by then bolting an L-bracket onto the top-plate of the y-stage and a further bearing stage onto the upright of the L-bracket, a three-axis stage, or so-called “xyz”-stage, is produced.
Sometimes multi-axis stages are supplied as integrated units. For example, in an xy-stage, the top plate of the x-stage can also serve as the base plate of the y-stage, to reduce size and weight. Flexure stages can also be nested in various ways to make them more compact. Additional features may accomplish rotational movement about an axis. In such circumstances, a maximum of six possible degrees of freedom is available: x, y and z linear movement and rotation about such axes (e.g., “pitch”, “yaw” and “roll”, respectively).
The integration of parts as described above does not affect the basic principle of operation which is to use a number of similar mechanisms connected serially. There are, however, several drawbacks to overcome in the use of a series of multi-axis stage assemblies, including the following:
1. The complexity and cost tends to increase with the number of axes;
2. The mass of moving parts increases with the number of axes, making the stage slower to respond;
3. The number of moving parts increases with the number of axes, making the stage assembly more susceptible to vibrations;
4. The force of repositioning a stage is transmitted through the preceding stages, causing disturbance, i.e. error, to the position of the stage assembly; and
5. The stiffness of the stage assembly decreases as the number of axes increases.
Additionally, undesirable deviations are amplified by the stacking of stages. This phenomenon is commonly known as “Abbe” error. The Abbe principle is the magnification of undesirable angular motion as the displacement of the workpiece from the undesirable angular motion increases.
One example which attempts to overcome some of these deficiencies is U.S. Pat. No. 4,694,477 to Siddall which is incorporated herein by reference. Siddall shows a stage apparatus with six degrees of freedom. The invention of Siddall has a single stage 1 supported vertically by three actuator assemblies 11, 13 and 15. The actuator assemblies 11, 13 and 15 consist of flexures 17, 19 and 21, respectively, coupling piezoelectric transducers to the stage 1 at three equiangular points. The three vertical actuator assemblies accomplish vertical linear motion and rotation about two perpendicular horizontal axes. Three similar horizontal actuator assemblies 37, 39 and 41 are attached to the stage 1 for accomplishing linear movement of stage 1 along two perpendicular horizontal axes. When the two parallel horizontal actuator assemblies 37, 39 are moved in opposite directions, rotation of stage 1 about the vertical axis is accomplished. Each of the pivot points for the pitch, roll and yaw motions is located within the stage assembly and the range of travel is limited.
Currently, the telecommunications network serving the United States and the rest of the world is evolving from analog to digital transmission with ever increasing bandwidth requirements for transmitting voice and data. Fiber optic cable is capable of carrying much more information than traditional copper cable. As a result, one method to increase bandwidth of telecommunications networks is realized by replacing copper cable with fiber optic cable. A large market for optoelectronic devices to supply the new technologies of high-speed communications has developed to meet this requirement. In tandem, a strong market has also developed for the sophisticated positioning apparatus required to manufacture such optoelectronic devices.
A key consideration in the design of fiberoptic networks is the effect of attenuation. Attenuation determines the maximum length of fiber that may be included between two points before it is necessary to include a signal repeater in the communication path that retransmits the signal. Attenuation occurs due to the light that carries the signal either leaking out of the fiber or being absorbed by the material from which the fiber is made. When the strength of the signal falls below a certain level as a result of this attenuation, the signal to noise ratio of the system may become too low for effective data transfer to be maintained and a repeater is required.
One known method to reduce the effects of attenuation on a transmitted signal is to increase the power of the light transmitted into the optical fiber. For a given amount of attenuation per length of fiber, increasing the power input to the fiber increases the distance that the fiber may be run before the signal level becomes unacceptably small. Of course, one way of increasing the power input to the fiber is to increase the power of the laser used to generate the light that is coupled to the fiber. Another way of increasing the strength of the signal carried by the fiber is to more efficiently couple, i.e. align, the modulated light from the laser into the fiber.
To meet such demand and efficiency requirements, the optoelectronic devices must be fabricated with a high degree of accuracy and in large quantity. In view of these needs, various techniques have been developed for applying positioning stage assemblies to align optical components. “Pigtailing” is the term commonly used to describe the process of aligning and attaching an optical fiber in front of an active optoelectronic device, such as a laser diode.
Additionally, pigtailing requires that the device and fiber be permanently coupled mechanically. Typically, this involves sub-micrometer accuracy, performed manually by skilled technicians working with microscopes and high-precision manipulators. In general, even though the step of coupling the light from the laser into a flat-end fiber is of critical importance, the process is not only time consuming but very inefficient, with many processes resulting in only about 10 to 15 percent of the laser light output being coupled into the fiber. Moreover, these techniques not only require skilled labor; but often, these techniques are more in the nature of an art which cannot easily, if at all, be transferred from one worker to another worker.
To elaborate with an example, the light coupled into or out of the optical fiber is highly sensitive to the alignment of the optical fiber with the laser or the detector and any optical system that is used between the optical fiber and such devices. A slight misalignment of the optical fiber may cause a large decrease in the amount of light coupled into the fiber from the laser or out of the fiber to the detector. In general, this problem is more serious at the laser end because the size of the emitting region of a typical laser diode used in such systems is approximately 2 microns by 4 microns. Similarly, the small aperture of a single mode optical fiber presents a significant technical challenge when aligning. Moreover, the use of focusing optics to focus light from the laser into the optical fiber also increases the sensitivity of the amount of coupling to the alignment of the cable with the source and any discrete optical devices used. In view of the above, once each optoelectronic device is assembled, testing is required to verify performance. Upon verification, the device is typically assembled onto a printed circuit board for connection to other electronic and optical signal processing components.
In view of the above, several techniques for coupling optical fibers to optical devices have been developed. Some examples are illustrated in U.S. Pat. Nos. 6,325,551; 6,253,011; 6,193,226; 6,174,092; 6,164,837; and 5,926,594, each of which is hereby incorporated by reference as part of the present disclosure.
Alignment of an optical fiber with an optoelectronic device may be accomplished by using a stage assembly to change the position of the optical fiber while measuring the amount of light coupled from the optical fiber to the optoelectronic device, e.g., “active” alignment. If, for example, the optoelectronic device is a laser diode, the light coming out of the other end of the optical fiber may be measured and the optical fiber may be positioned so that the amount of light output is maximized. Once the optical fiber is properly positioned, it is desirable to fix the alignment in a manner such that the alignment remains unchanged. Numerous methods of fixing optical fibers to supports also have been developed. These methods include epoxying or gluing the fiber to a mount, laser welding the fiber to a mount, and soldering the fiber to a mount. While a certain amount of success has been enjoyed using these prior art systems and methods for pigtailing, there are clear drawbacks and disadvantages with them.
Accordingly, it is an object of the present invention to provide a high-speed, high-precision instrument for the active alignment of fibers for the manufacture of optoelectronic devices. The improved instrument and method would preferably permit modification for varied specifications to minimize capital costs and aid in assuring adequate yield of product at a high throughput.
It is another object of the present invention to provide a system and method for actively aligning fibers in the manufacture of optoelectronic devices that overcomes one or more of the above-described drawbacks and disadvantages of the prior art.